Semiconductor transistors, in particular field-effect controlled switching devices such as Metal Oxide Semiconductor Field Effect Transistors (MOSFET) and Insulated Gate Bipolar Transistors (IGBT) have been used in a wide variety of applications such as power supplies, power converters, electric cars and air-conditioners. Many of these applications are high power applications, which require the transistors to be able to accommodate substantial current and/or voltage. In high power applications, two device parameters that play a substantial role in overall performance of the device are on-state resistance RON and breakdown voltage VBR. Lower on-state resistance RON is a desirable characteristic for a power transistor because it minimizes the resistive power loss (and corresponding heat generation) that occurs when the device is in a forward conducting state. Meanwhile, high breakdown voltage VBR is a desirable characteristic for a power transistor because ensures that the device will remain in an off-state under the presence of large reverse voltages.
Vertical transistors are commonly utilized in high power applications due to the favorable on-state resistance RON and breakdown voltage VBR characteristics that that these devices offer. Vertical devices are configured to conduct current in a direction perpendicular to the surfaces of the semiconductor substrate. Typically, these devices include a drift region in the substrate between the output regions (e.g., source/drain regions). By lowering the doping concentration of the drift region, the likelihood of avalanche breakdown in the device can be reduced and consequently the reverse blocking capability of the device can be improved. However, lowering the doping concentration of the drift region comes at the expense of an increased on-state resistance RON, because it lowers the concentration of carriers available for conduction when the device is in the on-state.
By improving the tradeoff between on-state resistance RON and breakdown voltage VBR, it is possible to lower the on-state resistance RON of the device while maintaining reverse blocking capability. Alternatively, an improvement to this tradeoff can be utilized to provide a device with increased reverse blocking capability while maintaining the on-state resistance RON of the device.
One technique that is utilized to favorably shift the tradeoff between on-state resistance RON and breakdown voltage VBR in a transistor involves taking advantage of the compensation principle. The compensation principle is based on a mutual compensation of charges in the device. Compensation structures can be provided at or near the drift region to produce opposite type carriers as those carriers that are present in a space charge region that forms in the drift region when the device is reverse biased.
One application of the compensation principle in power switching devices involves providing field plates in the device that vertically extend into the drift region. The field plates can be biased such that they introduce compensating charges into the drift region in a reverse-blocking state. However, field plates are not completely effective at eliminating the electric fields that cause avalanche breakdown.